Apple’s recently released iOS 8.3 update prevents desktop file explorers
like iFunBox, iExplorer, iTools, and others from accessing the app directories on your devices. A few have updated with temporary fixes, but it might take a little while before everything’s working again.

Essentially, iOS 8.3 prevents these utilities from accessing apps unless they have iTunes File Sharing enabled. This means you can’t manually transfer over files or back up save file information
. iFunBox explains the problem on their Facebook page:

OS 8.3 ruined our freedom of controlling data in our installed Apps and Games. Apple blocked access of the entire sandbox directory of every installed App since iOS 8.3. Previously only writing is block in executable directory. Now we totally lost the control of Apps on our own devices. We are investigating the situation. Before there is a solution, iFunbox will fail to open data directory of any App if the device is not jailbroken.

Which is all to say, if you use a desktop file explorer, you might have some issues transferring files back and forth until they figure it out. This is only causing problems for files inside apps though, so any other transfers (ring tones, pictures, whatever else) will continue to work.